The invention relates to a turbine nozzle made of composite material having a ceramic matrix or a matrix that is at least partially ceramic, and referred to below as CMC material.
The field of application of the invention is more particularly that of gas turbines for aviation or industry.
Improving the performance of gas turbines and reducing their polluting emissions is leading to ever-higher combustion temperatures being envisaged.
For the hot portions of a gas turbine, proposals have therefore been made to use CMC materials because of their remarkable thermostructural properties, i.e. because they associate mechanical properties that make them suitable for constituting structural elements with a capacity to conserve those mechanical properties at high temperatures. CMC materials are typically formed using fiber reinforcement made of refractory fibers, such as carbon fibers or ceramic fibers, and densified with a matrix that is ceramic or at least partially ceramic. As is well known, an interphase may be present between the fibers and the matrix in order to perform an embrittlement-release function for the composite material by deflecting cracks that reach the interphase. Such an interphase may be made of pyrolytic carbon (PyC), boron nitride (BN), or boron-doped carbon (BC). Embrittlement-release interphases of the same type may also be incorporated within the matrix, between phases of ceramic matrix. In any event, at least the outer phase of the matrix is ceramic.
Document EP 0 399 879 discloses a CMC turbine nozzle made up of a plurality of assembled-together portions, namely a hub carrying vanes and toothed rings that engage tenons formed at the radial ends of the vanes. The component parts of the turbine nozzle are obtained by machining solid CMC parts, thereby giving rise to large losses of material and leading to multiple ruptures of the fibers in the fiber reinforcement in the CMC.
Document EP 1 087 103 discloses a turbine nozzle element having a plurality of vanes extending between an inner annular platform sector and an outer annular platform sector. Each vane is made up of two portions: a CMC portion with braided fiber reinforcement beside the leading edge, and a solid ceramic portion beside the trailing edge. The platform sectors are made of CMC with fiber reinforcement built up from superposed plies. The braided reinforcement of the CMC portion of each vane extends beyond the longitudinal ends of the vane in order to be inserted between the fiber reinforcing plies of the platform sectors. The nozzle element is thus made in part only out of CMC and the fiber reinforcement is made up of a plurality of separate portions that need to be assembled together.
Document JP 2003-148 105 likewise discloses a nozzle element made of a single piece of ceramic matrix composite material. The fiber reinforcement is obtained in part from a woven blank in the form of a strip having a tubular central portion for forming a vane preform that is extended at each of its ends by flaps that face each other. The flaps are unfolded on either side of the central portion and they are stitched to separately-obtained fiber plies so as to form inner and outer platform preforms. The fiber structure occupying the entire volume of the nozzle element is thus made up of a plurality of separate portions that need to be assembled to one another.
Document WO 91/15357 discloses a turbine nozzle element made as a single piece out of ceramic matrix composite material. The fiber reinforcement is obtained from a blank in the form of a strip that may be woven, but that is preferably knitted. The strip is folded in half in order to obtain a fiber preform for the nozzle element. In the portion corresponding to a vane, the folding is performed in such a manner that the two edges of a segment of the strip take up juxtaposed positions and are connected together by stitching. There is thus discontinuity in the woven or knitted fiber reinforcement at the periphery of each vane along a line that extends in the longitudinal direction of the vane. Such a discontinuity is penalizing from a mechanical point of view.
Document EP 0 466 602 also discloses a single-piece nozzle element made of CMC obtained by densifying a fiber preform. The preform is obtained by shaping superposed plies or a three-dimensional woven texture. The nozzle element includes a vane with asymmetrical root portions and not with inner and outer platforms that extend on either side of the zone where they are connected to the vane.